Electric discharge tube comprising a directly heatable cathode

ABSTRACT

In a reticulated grid cathode which has a flexible connection at one end to the conductor for the filament current present inside the cathode, the flexible connection consists of a number of thin metal strips of the same thickness. The strips constitute an approximately semi-circular arc and each consist of two strips of preferably the same thickness. In order to minimize the bending stress occurring in the strips, the diameter of the arc formed by the strips is chosen to be as large as possible relative to the radial dimensions of the cathode.

United States Patent [191 Van Warmerdam 1 ELECTRIC DISCHARGE TUBECOMPRISING A DIRECTLY I-IEATABLE CATHODE [75] Inventor: .IoannesCornelis Van Warmerdam,

Emmasingel, Eindhoven,

Netherlands [73] Assignee: U.S. Philips Corporation, New

' York, NY.

[22] Filed: Feb. 2, 1973 [21] Appl. No.: 329,198

[30] Foreign Application Priority Data Feb. 17, 1972 Netherlands 7202069[52] US. Cl 313/341, 313/278, 313/348 [51] Int. Cl HOlj 1/15, I-I0lj19/08 [58] Field of Search 313/341, 278, 348

[56] References Cited UNITED STATES PATENTS 2,928,978 3/1960 Morton313/341 [451 Apr. 23, 1974 FOREIGN PATENTS OR APPLICATIONS 987,4613/1965 Great Britain 313/341 Primary Examiner-Archie R. BorcheltAssistant Examiner-Saxfield Chatmon, Jr. Attorney, Agent, or FirmFrankR. Trifari [5 7] ABSTRACT In a reticulated grid cathode which has aflexible connection at one end to the conductor for the filament currentpresent inside the cathode, the flexible connection consists of a numberof thin metal strips of the same thickness. The strips constitute anapproximately semi-circular arc and each consist of two strips ofpreferably the same thickness. In order to minimize the bending stressoccurring in the strips, the diameter of the are formed by the strips ischosen to be as large as possible relative to the radial dimensions ofthe cathode.

4 Claims, 3 Drawing Figures PATENTEDAPR 23 m4 3" sum 1 BF 2 ELECTRICDISCHARGE TUBE COMPRISING A DIRECTLY HEATABLE CATHODF,

The invention relates to an electric discharge tube comprising acylindrical directly heatable cathode which is constructed from wires ofthoriated tungsten or another high-melting-point metal which are woundaccording to left and according to right helical lines and extend oneither side of a same cylinder surface, said wires being welded togetherat least at a number of the intersections, said cathode being rigidlyconnected at one end to a supporting cylinder which extends coaxiallywith the cathode and being connected at its other end to a metalconductor present inside the cathode by means of a number of thin metalstrips extending radially but axially at the ends.

The above-described cathodes constructed from metal wires are known bythe name of reticulated grid cathodes. It is known from the U.S. Pat.No. 2,928,978 to connect one end of such a cathode by means of flexiblestrips to the current conveying conductor present inside the cathode. Anobject of this flexible connection is to prevent that the reticulatedgrid cathode is deformed by thermal stresses resulting from the laggingof the variations in length of the supporting cylinder relative to thatof the cathode when the filament current is switched on and off. Forthat purpose it should be possible for the thermal expansion of thecathode to take place in an unhindered manner in an axial directionwhich requires the connection between the cathode and the said conductorto be extremely flexible and thus to be formed from strips which are asthin as possible. On the other hand, said strips may not be too thinsince they are traversed by the filament current for the cathode andthus should have a minimum electric resistance.

The cathode of the above-described patent has a length of approximately6 cm and a diameter of approximately cm. The flexible strips whichconnect said cathode to the interior conductor have an S- shaped bentportion. Owing to the small distance between the conductor and thecathode, the strips are very sharply bent, that is to say over a veryshort distance. As a result of this, the flexibility of the constructionin the axial direction is comparatively small although this will hardlybe a drawback for cathodes of the above-described dimensions. Incathodes, however, having a length of more than 200 mm, much higher requirements are imposed upon the flexibility of the suspensionconstruction and the known construction can no longer be used.

Deformation of the cathode at the operating temperature as a result ofits weight can be prevented by a correct choice of the length andthickness of the cathode wires, the number of welded intersections, thediameter of the cathode and the number of cathode wires. The problem isdefinitely more difficult to obtain a connection between the cathode andthe inside conductor which in addition to a sufficient flexibility alsohas a very low electric resistance.

From U.S. Pat. No. 3,560,791 a construction is known the flexibleconnection of which between the cathode and the central conductor isformed by a cap which is constructed from a number of thin circulardishes which are stacked one on the other and are rotated through agiven angle relative to each other, said dishes each comprising leafsprings which extend radially towards the cathode and are bent at rightangles at that area. The rigidity of such a construction, however, isstill too large to use it in the reticulated grid cathode in questionhaving a length of more than 200 mm. Another drawback of thisconstruction is in addition that, taken in the radial direction to thecenter of the dishes, the leaf springs enclose each other for an everincreasing part, as a result of which the thermal dissipation at thatarea is impeded to a considerable extent.

It is an object of the invention to provide a construction of the typedescribed in the preamble in which the connection between one end of thereticulated grid cathode and the interior conductor has a low electricresistance, while nevertheless the flexibility in the axial direction isso large that deformation of the cathode as a result of the thermalstresses when the filament current is switched on and off is minimized.

In a cylindrical reticulated grid cathode which is flexibly connected atone end to a metal conductor present centrally inside the cathode and inwhich said connection is formed by a number of thin metal strips ofequal thickness extending radially but axially at the ends, according tothe invention the two ends of the said strips extend over a restrictedlength in the same sense in the axial direction and the part presentbetween said ends constitutes an approximately semi-circular arc thediameter of which, measured between the two parallel ends, is equal to0.2 to 0.4 times the diameter of the cylindrical cathode.

The construction according to the invention has particularly favourableproperties. The shape in which the strips are bent give the constructiona great flexibility in the axial direction. Another advantage of theconstruction according to the invention over that mentioned in the abovedescribed patent specifications is that when the length of the cathodevaries, the strips roll up and down on the elongation of the connectionsurfaces during which movement radial forces hardly occur, provided theradius of curvature of the are formed by the strips is chosen to be aslarge as possible relative to the radial dimensions of the cathode. Inthat case, actually, the bending moment occurring in the strips is assmall as possible. The upper limit imposed upon said radius of curvaturewith a given cathode diameter depends upon the required number of stripswhich must be capable of terminating on the circumference of thefilament current conductor present centrally inside the cathode.Actually, with a given width of the strips, the minimum number is fixedwith the electric resistance still just permissible which is representedby the parallel arranged strips.

A reduction of the electric resistance of the strips, without the saidfavourable properties of the construction being lost, is realised if thestrips, taken from the narrow end secured to the cylinder having thesmaller diameter, change into a part having a larger width via a gradualtransistion.

Since the rigidity of the strips is proportional to the third power ofthe thickness of the strips, it is particularly favourable to composeeach individual strip from two sub-strips or strip elements ofpreferably the same thickness. The flexibility then becomes better byapproximately a factor four, while the electric resistance of the stripsdoes not vary. A construction of each individual strip from more thantwo such strip elements has the drawback that the heat dissipation ofthe strips enclosing each other on either side would be too stronglyimpeded.

The invention will be described in greater detail with reference to thedrawing, of which FIG. 1 is a perspective view of an embodiment of asuspension construction for a cathode according to the invention;

FIG. 2 is a longitudinal sectional view of the cathode suspension ofFIG. 1, and

FIG. 3 shows a strip in the stretched condition.

The reticulated grid cathode 1 shown diagrammatically in FIG. 1 isconstructed from 30 wires of thoriated tungsten extending according to aleft helical line and 30 wires extending according to a right helicalline, said wires being welded together at the intersections. The cathodewires have a thickness of 0.45 mm and constitute a cathode having alength of 257 mm and a diameter of 78.8 mm. At one end, the cathode 1 iswelded to the outer ring of a circular rectangular metal channel 2 andat its other end to a circular ring 3, said ring constituting one end ofthe hollow metal supporting cylinder 4. Inside the hollow cylinder 4 andthe cathode 1, the hollow metal cylinder 5 extends coaxially and formspart of the filament current circuit of the cathode and changes via adish-like member 6 into a hollow cylinder 7 of smaller diameter. Theholes 11 present in the cylinder 5 provide access to a fewnonevaporating getters present behind said holes. The thin molybdenumstrips 8 are secured to the free end of the cylinder 7 and clampedbetween the cylinder wall and the strip 9 which likewise consists ofmolybdenum. From this connection the strips 8 extend initially in theaxial direction, then describe an approximately semicircular arc andfinally terminate again in the axial direction between the molybdenumstrip 10 and the inner ring of the cathode channel 2.

In order to improve the flexibility of the construction, each individualstrip consists of two strips which overlap each other entirely and haveeach a thickness of 0.1 mm.

FIG. 2 is a longitudinal sectional view of the suspension constructionfor the cathode of FIG. 1. The strips 8 consist of two strips 12 and 13which each have a thickness of 0.1 mm. The connection between thecathode 1 and the conductor 7 is constituted by 16 such compositestrips. The diameter of the are formed by the strips, measured betweenthe parallel end members thereof, is 23 mm, while the supportingcylinder 7 has a diameter of 22 mm and the cathode 1 a diameter of Theconstruction according to the invention furthermore consists of ascreening cap 14 which for clarity of the drawing is not shown inFIG. 1. By means of the to the metal sleeve 15. The sleeve 15 is weldedto the cylinder 7 at 18. The screening cap 14 is at cathode potentialand prevents any electron emitted by the strips 8 and contributing to anundesired anode dissipation from reaching the anode.

FIG. 3 shows a tape 8 in the stretched condition. At 21 the strip has aportion which narrows over a length of 12 mm which portion changes at 22into a section having a length of 28 mm and a width of 6 mm and at 23changes into a section having a length of 8 mm and a width of 3.5 mm.

1. An electric discharge tube, comprising:

a. a support cylinder;

b. a cylindrical directly heatable cathode which is constructed fromwires of a high-melting-point metal, said wires being wound according toleft and according to right helical lines and extending on either sideof a common cylinder surface defined thereby, said wires being weldedtogether at least at a number of the intersections therebetween, saidcathode being rigidly connected at one end to said support cylinder andextending coaxially with said support cylinder;

c. a metal conductor present inside said cathode; and

d. plural thin metal strips extending radially between and connectingsaid metal conductor and the other end of said cathode, said stripsextending axially at the respective ends thereof, end portions of eachof said strips extending in substantially the same axial direction andthe part of each said strip present between said ends thereofconstituting a substantially 3. An electric dicharge tube as claimed inclaim 1, wherein said strips individually have a narrow portion, a wideportion and an intermediate transition portion which gradually mergessaid narrow and wide, said wide portion being disposed at said cathodegrid and said narrow portion being disposed interiorly thereto.

4. An electric discharge tube as defined in claim 2, wherein said stripelements are substantially equal in thickness.

t l l t t

1. An electric discharge tube, comprising: a. a support cylinder; b. acylindrical directly heatable cathode which is constructed from wires ofa high-melting-point metal, said wires being wound according to left andaccording to right helical lines and extending on either side of acommon cylinder surface defined thereby, said wires being weldedtogether at least at a number of the intersections therebetween, saidcathode being rigidly connected at one end to said support cylinder andextending coaxially with said support cylinder; c. a metal conductorpresent inside said cathode; and d. plural thin metal strips extendingradially between and connecting said metal conductor and the oTher endof said cathode, said strips extending axially at the respective endsthereof, end portions of each of said strips extending in substantiallythe same axial direction and the part of each said strip present betweensaid ends thereof constituting a substantially semi-circular arc havinga diameter as measured between said parallel end portions, equal toabout 0.2 to about 0.4 times the diameter of said cylindrical cathode.2. An electric discharge tube as claimed in claim 1, wherein each ofsaid strips consists of two strip elements which substantially entirelyoverlap each other.
 3. An electric dicharge tube as claimed in claim 1,wherein said strips individually have a narrow portion, a wide portionand an intermediate transition portion which gradually merges saidnarrow and wide, said wide portion being disposed at said cathode gridand said narrow portion being disposed interiorly thereto.
 4. Anelectric discharge tube as defined in claim 2, wherein said stripelements are substantially equal in thickness.